Unprecedented Precision in Earth’s Rotation Measurement Using Ring Laser Technology

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Researchers have achieved a groundbreaking milestone in the precise measurement of Earth’s rotation by employing an advanced ring laser system at the Geodetic Observatory Wettzell. This technological advancement enables the acquisition of daily high-quality data, a critical factor in determining Earth’s spatial position and enhancing climate research and modeling. (Illustration: Depicting the application of laser technology for Earth’s rotation measurement.)

Scientists from the Technical University of Munich (TUM) have made remarkable progress in accurately measuring Earth’s rotation. The ring laser system at the Geodetic Observatory Wettzell now offers unparalleled data quality, unrivaled anywhere else in the world. These measurements play a pivotal role in pinpointing Earth’s location in space, contributing to climate research, and elevating the precision of climate models.

Advanced Ring Laser Technology

Have you ever wondered about the speed at which Earth rotates in the past few hours? Now, at the Geodetic Observatory Wettzell, you can witness this phenomenon firsthand. TUM researchers have upgraded the ring laser system, allowing it to provide up-to-date data on a daily basis, a level of quality previously unattainable.

But what does this ring laser system measure exactly? As Earth journeys through space, its rotation on its axis varies slightly in speed. Moreover, the axis around which the planet spins experiences subtle oscillations. This is because our planet is not entirely solid, comprising various components, some solid and some liquid. Consequently, the Earth’s interior is in constant motion, influencing the planet’s rotation, and these variations can be detected through measurement systems such as the TUM ring laser.

The ring laser at Wettzell has undergone continuous refinement since its inception. Credit: Astrid Eckert / TUM

“Fluctuations in Earth’s rotation are not only of astronomical interest but are also crucial for precise climate modeling and a deeper understanding of weather phenomena like El Niño. The accuracy of predictions depends on the precision of data,” explains Prof. Ulrich Schreiber, who led the project at the Observatory for TUM.

Technical Enhancements and Challenges

During the overhaul of the ring laser system, the team focused on striking a balance between size and mechanical stability. Larger devices can make more sensitive measurements, but size also introduces compromises in stability and, subsequently, precision.

Another challenge involved achieving symmetry in the two opposing laser beams, the core of the Wettzell system. Exact measurements are only possible when the waveforms of these two counter-propagating laser beams are nearly identical. However, the system’s design inherently introduces a degree of asymmetry. Over the past four years, geodesists have employed a theoretical model for laser oscillations to successfully capture these systematic effects, allowing for precise long-term calculations and their subsequent elimination from measurements.

Enhanced Precision and Applications

With the help of this new corrective algorithm, the system can now measure Earth’s rotation accurately to nine decimal places, equivalent to a fraction of a millisecond per day. In terms of laser beams, this corresponds to an uncertainty starting at only the 20th decimal place of the light frequency and remains stable for several months. Overall, the observed fluctuations in rotation reached values of up to 6 milliseconds over approximately two weeks.

These improvements have significantly reduced measurement periods as well. The newly developed corrective programs enable the team to capture current data every three hours. According to Urs Hugentobler, Professor for Satellite Geodesy at TUM, “In geosciences, such high time resolution levels are groundbreaking for standalone ring lasers. Unlike other systems, our laser operates independently and doesn’t rely on celestial observations or satellite data for reference points. This independence and precision are crucial.” Data collected independently of celestial observations can help identify and correct systematic errors in other measurement methods. Further enhancements to the system, enabling even shorter measurement periods, are planned for the future.

Understanding Ring Lasers

Ring lasers consist of a closed square beam path with four mirrors entirely enclosed in a Ceran glass-ceramic body known as the resonator. This design ensures that the path length remains constant despite temperature fluctuations. A mixture of helium and neon gas within the resonator facilitates the excitation of laser beams, one moving clockwise and the other counterclockwise.

In the absence of Earth’s movement, both laser beams would travel the same distance in both directions. However, since the device rotates with the Earth, one of the laser beams covers a shorter distance due to Earth’s rotation bringing the mirrors closer to the beam. In the opposite direction, the light travels a correspondingly longer distance. This effect results in a difference in the frequencies of the two light waves, creating a precisely measurable beat note. The greater the speed of Earth’s rotation, the larger the difference in the optical frequencies. At the equator, where Earth rotates at 15 degrees per hour to the east, this generates a signal of 348.5 Hz in the TUM device. Fluctuations in the length of a day manifest as values ranging from 1 to 3 millionths of a Hz (1 – 3 microhertz).

Robust and Precise Infrastructure

Each side of the ring laser system in the basement of the Wettzell Observatory spans four meters. This construction is firmly anchored to a solid concrete column resting on the Earth’s bedrock, approximately six meters below the surface. This design ensures that Earth’s rotation is the only factor affecting the laser beams, eliminating the influence of other environmental factors. The construction is further safeguarded by a pressurized chamber, which automatically compensates for changes in air pressure and maintains a stable temperature of 12 degrees Celsius. To minimize external factors, the laboratory is situated five meters below an artificial hill. Nearly two decades of research have gone into the development of this measurement system.

Reference: “Variations in the Earth’s rotation rate measured with a ring laser interferometer” by K. Ulrich Schreiber, Jan Kodet, Urs Hugentobler, Thomas Klügel, and Jon-Paul R. Wells, published on September 18, 2023, in Nature Photonics.
DOI: 10.1038/s41566-023-01286-x

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More about Earth’s Rotation Measurement

• Technical University of Munich (TUM)
• Geodetic Observatory Wettzell
• Nature Photonics Article: “Variations in the Earth’s rotation rate measured with a ring laser interferometer”